This invention relates generally to a method and apparatus for broadband decoupling scalar or dipolar couplings between nuclei in a sample by inverting a longitudinal magnetization in the sample with cycles of chirp pulses.
By way of background, broadband decoupling of scalar or dipolar couplings with spins having I=1/2 is one of the central challenges of nuclear magnetic resonance. As greater static magnetic fields are becoming available, the bandwidths that must be covered are increasing. For example, in a 1000 MHz spectrometer equipped with a 23.5T magnet, a bandwidth of 50 kHz is required for decoupling carbon-13 spectra of 200 ppm width. When working with conducting aqueous solutions of biological macromolecules, it is also desirable to limit the average radio frequency power to prevent sample heating. For in vivo decoupling with surface coils, decoupling methods must also be very tolerant to RF inhomogeneity.
In the past, noise decoupling has been found to be relatively inefficient and the use of singular chirp pulses has not been successful over a broad band. Other efforts, such as MLEV, WALTZ and GARP, have concentrated on sequences of phase-shifted rectangular pulses where the carrier frequency is kept constant. These sequences are derived from a combination of elements R organized into cycles and supercycles for inverting the longitudinal magnetization over as wide a bandwidth as possible. In MPF schemes, the constraint that the carrier frequency remain constant is dropped and the carrier is stepped through the spectrum in large frequency increments.
More recent broadband decoupling schemes have been built on adiabatic inversion where the carrier frequency is varied smoothly using a hyperbolic secant pulse shape in combination with 4-step and 5-step supercycles. However, these nonlinear schemes require relatively high RF amplitudes over fairly narrow bandwidths.